In recent years, an audio signal demodulation circuit has been regarded as important as a circuit for demodulating PCM sound (audio signals) which is indispensable to high definition television broadcasting.
Now referring to the drawings, one example of the conventional audio signal demodulation circuits will be explained.
FIGS. 4, 5 and 6 are a block diagram of the conventional audio signal demodulation circuit, a timing chart thereof and a waveform chart thereof, respectively. In FIG. 4, 1 is a bit de-interleave circuit; 2 is an error correction circuit; 3 is a word de-interleave circuit; 4 is an expansion circuit; 5 is a range detection/error correction circuit; 6 is an interpolation circuit; 7 is an integration circuit; 8 is a synchronous detection circuit; and 9 is an AND circuit.
The operation of the audio signal demodulation circuit thus constructed will be explained with reference to FIGS. 5 and 6.
First, an audio input signal at 1350 kb/s is detected in its synchronization pattern by the synchronous detection circuit 8 to take synchronization. The output (FIG. 5(E)) from the synchronous detection circuit 8 is at a high level if the synchronization has been taken, while it is at a low level for a muting operation if the synchronization has not been taken.
Also the bit interleave of the input audio signal performed on the transmission side is de-interleaved by the bit de-interleave circuit 1. The output from the bit de-interleave circuit 1 is error-corrected by the error correction circuit 2 so that one-error correction two error detection is made in the normal mode and two-error correction three-error detection is made in the intensifying mode. The signal ((B) of FIG. 5) subjected to the two-error detection in the normal mode and the three-error detection in the intensifying mode will be used as an interpolation signal in the interpolation circuit 6.
The word interleave of the output of the error correction circuit 2 performed on the transmission side is de-interleaved by the word de-interleave circuit 3.
The output from the error correction circuit 2 is also detected in its range bits and error-detected by the range detection/error correction circuit 5.
The outputs from the word de-interleave circuit 3 and the range detection/error correction circuit 5 are expanded by the expansion circuit 4 using the range bits to produce a differential value. The output ((A) of FIG. 5) from the expansion circuit 4 is interpolated by the interpolation circuit 6 using the interpolation signal ((B) of FIG. 5) as follows: ##EQU1##
In the case where there are successive interpolation signals consisting of n (integer) samples from X.sub.(i+1) to X.sub.(i+n) for a certain channel, the n samples containing two errors in the normal mode and three errors in the intensifying mode are interpolated so that (n-1) samples are interpolated for the previous value X.sub.(i) and the last sample are interpolated for the average value of X.sub.(i) and X.sub.(i+n+1) which contain no error ((C) of FIG. 5).
The output ((C) of FIG. 5) from the interpolation circuit 6 is muted by the AND circuit 9 using the mute signal ((E) of FIG. 5) output from the synchronous detection circuit so that the differential value when the synchronization has not been taken is zero ((H) of FIG. 5).
This differential value (H) from the AND circuit 9 is integrated by the integration circuit 7 to be demodulated as an audio signal.
The above arrangement thus constructed has the following defect. If three errors and two errors successively occur in the normal mode and the intensifying mode, respectively, the previous value is used for interpolation of the differential value. This increases distortion of the sound obtained and so greatly deteriorates the sound quality; the sound thus obtained cannot be heard as suitable sound by audiences.